Quantification of the effect of road geometry and environment on vehicle speed

Abstract

Characteristics of drivers and vehicles, elements of road geometry and environmental conditions are the major factors affecting vehicles speed. The first two items can be identified but are difficult to be quantified. Road geometry and environmental conditions are apt to be both identified and quantified. In this study six important factors which have significant influence on vehicles speeds, such as road deterioration, curvature, presence of motorized and non-motorized vehicles in the traffic stream as well as environmental conditions such as dry/wet pavement conditions, visibility etc. have been identified for investigation. The main objectives of this study was to quantify the effect of these factors on vehicles speeds and to identify the factors which affect vehicles speed most. In order to quantify the effect of a particular factor on speeds, comparative studies use carried out between speeds in ideal road conditions and that of road with the infl uence of the particular factor being studied. Spot speeds data were collected manually from nine roads links in Dhaka city road network. Computer program EXCEL has been used for comprehensive statistical analysis viz. mean speed, speed range, standard deviation, maximum and minimum speeds values etc. for each type of vehicle categories. ORIGIN software has been used for producing bar chart to show the effects of selected variables on vehicles speeds. This research has revealed that at free flow condition in urban area width of road has significant effect on free speed of passenger cars and for this type of vehicle the maximum reduction of speed was observed to be around 30% when speeds were compared between twelve and four lane road. The effect of road width on the mean speed of cycle was found to be a minimum reduction of nearly 7.7%. From four lanes road to six lanes road the mean stream speed of motorized vehicles is increased only by 3.6 km/hr in built-up area and for twelve lanes this value is increased by 16 kmlhr. Medium rainy condition was found to affect considerably the mean speed of micro buses, utilities, mini buses, auto rickshaws and motor cycles. The reduction were of the order of 44.4%, 39.8%, 34.3%, 24.2% and 44.4% respectively. Rainy condition decreased the mean stream speed for motorized of vehicles by 29.5%. The result of this research has shown that particular deterioration has maximum effect on the mean speeds of large bus and bi-cycle with speed reduction of 35.4% and 21.7% respectively. Deterioration of road decreases the motorized stream speed by 28.6%. At free flow condition the effect of Non-Motorized Vehicles (NMV) is found to have the minimum effect on the mean speeds of passenger cars, util ity vehicles, mini buses, large buses, auto rickshaw and motor cycles. The minimum effect on the mean speed of micro bus is found from the radius of curve at 145 ft and its mean speed is decreased by 4.4%. For other types of vehicles, due to the effect of curvature mean speeds were decreased by 5% to 20%. The effect of curvature on motorized stream speed is very low. Results indicate that the mean speed of motorized vehicles at night time decreased by 10%-25% of day time speed values except for auto rickshaw. One interesting finding was cycle rickshaw speeds showed slight increases at night time. This condition may be due to the fact that, rickshaw pullers feel more comfortable 10 pull rickshaws at cooler temperature than the hot sunny day when they get tried very quickly. At night the mean stream speed of motorized traffic is decreased by 6 to 7 kmlhr and non-motorized stream speed is increased by 2 kmlhr. It is also revealed that at free flow condition mean speeds of the loaded rickshaws are slightly greater than those of the unloaded rickshaws. From the analyses of mean speeds, standard deviations and speed ranges it was revealed that by and large the mean free speeds of auto rickshaws and trucks were not affected significantly by road geometries and environmental conditions except highly deterioration effect for Auto rickshaws. The mean speed of auto rickshaw decreased about 20% on highly deteriorated roads and for other conditions, auto rickshaws were found that it run at their maximum speed levels and were found to be changing less sensitive to roadway conditions.